This research proposal is composed of three interrelated projects: 1) the ionic mechanisms of the plateau of the action potential in frog ventricle using gap voltage clamp technique. This study is being carried out in three phases; a) development and perfection of guard-gap voltage clamp technique, making measurement of true membrane current in multi-cellular tissues possible; b) the nature of rectifying characteristics of myocardial cell during the plateau; c) finally, separation of the ionic components of the plateau currents. 2) The electromechanical and electrophysiological characteristics of uni-layered heart obtained from tunicate, Boltenia ovifera. The perfusion of this myocardial tube and its voltage clamping accompanied with tension measurements will be undertaken. A similar technique to that employed on the giant axon of squid will be employed. The ionic nature and a possible carrier-mediated transport of Ca 2 ion and K ion will be studied in this uni-layered heart without inherent complications caused by extracellular unstirred layers or inhomogeneity in clamped potential due to anamolous distribution of current. 3) An understanding of the nature of oscillatory pacemaker activities in the pacemaker cells. The role of catecholamines and cyclic AMP derivatives in generation of pacemaker activity is being investigated in pacemaker cells of sinus venosus obtained from frog hearts.